The printed circuits utilized in the electronic industry consist of a plastic laminate optionally reinforced with electrically resistant glass fibers. Connectors are placed on one side or both sides of such laminate. The connectors are made of flat strips of a conductive metal, usually copper. They serve to connect with one another the electronic components arranged on one side of the printed circuit.
The connection between the above connectors and the electronic components is obtained by welding or soldering.
Welding or soldering of the printed circuits is effected after having coated the whole circuit with a flux consisting of a resin containing activating additives such as, for example, amine hydrochlorides.
The flux prevents or removes the corrosion of the conductive material by promoting welding adhesion.
After the welding process, which causes a thermal degradation of part of the flux, the flux residues are removed using organic solvents.
The chlorofluorocarbons, due to their low toxicity and inflammability, prove to be the most suitable solvents for such utilization.
On the other hand, however, these solvents do not exhibit an adequate capability of dissolving the flux.
To enhance the dissolving power of the chlorofluorocarbons towards the flux residues, a mixture with more active solvents, such as e.g. alcohols with a low number of carbon atoms, has been proposed.
In particular, because the mixture is utilized at boiling, the alcohols are present in the solvent mixture in such concentration as to form an azeotrope with the chlorofluorcarbon.
However, with this solvent composition, because the printed circuit cleaning occurs in hot conditions, while the solubility of the flux in the solvent increases, the alcohol contained in the mixture gives rise, at the same time, to corrosion of Al, Sn and Pb, Zn, Cu, Ni and Fe metal surfaces.
By consequence, low nitromethane concentrations have been added to the above azeotropic compositions.
U.S. Pat. No. 3,903,009, hereby incorporated by reference, describes an azeotropic minimum composition containing 95.3% by weight of 1,1,2-trichloro-1,2,2-trofluoroethane, 3.6% by weight of ethanol and 1.1% by weight of nitromethane. This composition is useful to clean printed circuits.
However, this composition exhibits several drawbacks.
After application of the composition, white residues, due to the flux corrosion, remain on the printed circuit.
On the other hand, this solvent mixture must be used in hot conditions. From an industrial viewpoint, that requires more complicated cleaning apparatus equipped with heaters, and therefore higher energy consumptions.
Furthermore, although such mixtures are inflammable, they can decompose when coming into contact with hot surfaces. This generates toxic and corrosive vapors, which are dangerous for plant operators. At the same time, because it is practically impossible to separate the generated decomposed vapors from the ones of the fluxing mixture, this results in corrosion of the metal surfaces.
Therefore, it is a necessity to find mixtures for defluxing the printed circuits, which mixtures are capable of securing a thorough removal of the flux and which, at the same time, do not give rise to the mentioned drawbacks.